The present invention relates, generally, to methods and apparatus for making holograms, and more particularly to a technique for sequentially exposing a film substrate to a plurality of two-dimensional images representative of a three-dimensional physical system to thereby produce a hologram of the physical system.
A hologram is a three-dimensional record, e.g., a film record of a physical system which, when replayed, produces a true three-dimensional image of the system. Holography differs from stereoscopic photography in that the holographic image exhibits full parallax by affording an observer a full range of viewpoints of the image from every angle, both horizontal and vertical, and full perspective, i.e. it affords the viewer a full range of perspectives of the image from every distance from near to far. A holographic representation of an image thus provides significant advantages over a stereoscopic representation of the same image. This is particularly true in medical diagnosis, where the examination and understanding of volumetric data is critical to proper medical treatment.
While the examination of data which fills a three-dimensional space occurs in all branches of art, science, and engineering, perhaps the most familiar examples involve medical imaging where, for example, Computerized Axial Tomography (CT or CAT), Magnetic Resonance (MR), and other scanning modalities are used to obtain a plurality of cross-sectional images of a human body part. Radiologists, physicians, and patients observe these two-dimensional data xe2x80x9cslicesxe2x80x9d to discern what the two-dimensional data implies about the three-dimensional organs and tissue represented by the data. The integration of a large number of two-dimensional data slices places great strain on the human visual system, even for relatively simple volumetric images. As the organ or tissue under investigation becomes more complex, the ability to properly integrate large amounts of two-dimensional data to produce meaningful and understandable three-dimensional mental images may become overwhelming.
Other systems attempt to replicate a three-dimensional representation of an image by manipulating the xe2x80x9cdepth cuesxe2x80x9d associated with visual perception of distances. The depth cues associated with the human visual system may be classified as either physical cues, associated with physiological phenomena, or psychological cues, which are derived by mental processes and predicated upon a person""s previous observations of objects and how an object""s appearance changes with viewpoint.
The principal physical cues involved in human visual perception include: (1) accommodation (the muscle driven change in focal length of the eye to adapt it to focus on nearer or more distant objects); (2) convergence (the inward swiveling of the eyes so that they are both directed at the same point); (3) motion parallax (the phenomenon whereby objects closer to the viewer move faster across the visual field than more distant objects when the observer""s eyes move relative to such objects); and (4) stereo-disparity (the apparent difference in relative position of an object as seen by each eye as a result of the separation of the two eyes). The principal psychological cues include: (1) changes in shading, shadowing, texture, and color of an object as it moves relative to the observer; (2) obscuration of distant objects blocked by closer objects lying in the same line of sight; (3) linear perspective (a phenomenon whereby parallel lines appear to grow closer together as they recede into the distance); and (4) knowledge and understanding which is either remembered or deduced from previous observations of the same or similar objects.
The various psychological cues may be effectively manipulated to create the illusion of depth. Thus, the brain can be tricked into perceiving depth which does not actually exist. However, the physical depth cues are not subject to such manipulation; the physical depth cues, while generally limited to near-range observation, accurately convey information relating to an image. For example, the physical depth cues are used to perceive depth when looking at objects in a small room. The psychological depth cues however, must be employed to perceive depth when viewing a photograph or painting (i.e. a planar depiction) of the same room. While the relative positions of the objects in the photograph may perhaps be unambiguously perceived through the psychological depth cues, the physical depth cues nonetheless continue to report that the photograph or painting is merely a two-dimensional representation of a three-dimensional space.
Stereo systems depend on image pairs each produced at slightly different perspectives. The differences in the images are interpreted by the visual system (using the psychological cues) as being due to relative size, shape, and position of the objects and thus create the illusion of depth. A hologram, on the other hand, does not require the psychological cues to overrule the physical depth cues in order to create the illusion of a three-dimensional image; rather, a hologram produces an actual three-dimensional image.
Conventional holographic theory and practice teach that a hologram is a true three-dimensional record of the interaction of two beams of coherent, i.e. mutually correlated light, in the form of a microscopic pattern of interference fringes. More particularly, a reference beam of light is directed at the film substrate at a predetermined angle with respect to the film. An object beam, which is either reflected off of or shines through the object to be recorded, is generally normally (orthogonally) incident to the film. The reference and object beams interact within the volume of space occupied by the film and, as a result of the coherent nature of the beams, produce a standing (static) wave pattern within the film. The standing interference pattern selectively exposes light sensitive elements within the photographic emulsion comprising the film, resulting in a pattern of alternating light and dark lines known as interference fringes. The fringe pattern, being a product of the standing wave front produced by the interference between the reference and object beams, literally encodes the amplitude and phase information of the standing wave front. When the hologram is properly re-illuminated, the amplitude and phase information encoded in the fringe pattern is replayed in free space, producing a true three-dimensional image of the object.
Conventional holographic theory further suggests that a sharp, well defined fringe pattern produces a sharp, bright hologram, and that an overly strong object beam will act like one or more secondary reference beams causing multiple fringe patterns to form (intermodulation) and diluting the strength of the primary fringe pattern. Accordingly, holographers typically employ a reference beam having an amplitude at the film surface approximately five to eight times that of the object beam to promote the formation of a single high contrast pattern within the interference fringe pattern and to reduce spurious noise resulting from bright spots associated with the object. Moreover, since known holographic techniques generally surround the recording of a single hologram or, alternatively, up to two or three holograms, within a single region of the emulsion comprising film substrate, the stated objective is to produce the strongest fringe pattern possible to ensure the brightest holographic display. Accordingly, holographers typically attempt to expose a large number of photosensitive grains within the film emulsion while the object is being exposed. Since every point within the holographic film comprises part of a fringe pattern which embodies information about every visible point on the object, fringe patterns exist throughout the entire volume of the film emulsion, regardless of the configuration of the object or image which is the subject of the hologram. Consequently, the creation of strong, high contrast fringe patterns necessarily results in rapid consumption of the finite quantity of photosensitive elements within the emulsion, thereby limiting the number of high contrast holograms which can be produced on a single film substrate to two or three. Some holographers have suggested that as many as 10 to 12 different holographic images theoretically may be recorded on a single film substrate; however, heretofore, superimposing more than a small finite number of holograms has not been recognized and, in fact, has not been possible in the context of conventional hologram theory.
In prior art holograms employing a small number of superimposed holographic images on a single film substrate, the existence of a relatively small percentage of spurious exposed and/or developed photosensitive elements (fog) does not appreciably degrade the quality of the resulting hologram. In contrast, holograms made in accordance with the subject invention, discussed below, typically employ up to 100 or more holograms superimposed on a single film substrate; hence, the presence of a small amount of fog on each hologram would have a serious cumulative effect on the quality of the final product.
A method and apparatus for producing holograms is therefore needed which permits a large number, for example several hundred or more different holograms, to be recorded on a single film substrate, thereby facilitating the true, three-dimensional holographic reproduction of human body parts and other physical systems which are currently viewed in the form of discrete data slices.
The present invention provides methods and apparatus for making holograms which overcome the limitations of the prior art.
In accordance with one aspect of the present invention, a hologram camera assembly comprises a single laser source and a beam splitter configured to split the laser beam into a reference beam and an object beam and to direct both beams at a film substrate. The assembly further comprises a spatial light modulator configured to sequentially project a plurality of two-dimensional images, for example a plurality of slices of data comprising a CT scan data set, into the object beam and onto the film. In this manner, a three-dimensional holographic record of each two-dimensional slice of the data set is produced on the film.
In accordance with another aspect of the invention, the entire data set, consisting of one to two hundred or more individual two-dimensional slices, is superimposed onto the film, resulting in the superposition of one hundred or more individual, interrelated holograms on the single substrate (the master hologram). In contrast to prior art techniques wherein a small number (e.g., one to four) of holograms are superimposed onto a single film substrate, the present invention contemplates methods and apparatus for recording a large number of relatively weak holograms, each consuming an approximately equal, but in any event proportionate, share of the photosensitive elements within the film.
In accordance with a further aspect of the invention, a copy (transfer) assembly is provided whereby the aforementioned master hologram may be quickly and efficiently reproduced in a single exposure as a single hologram.
In accordance with yet a further aspect of the invention, a reference to object beam ratio of approximately unity is employed in making the master hologram, thereby conserving the number of photo-sensitive elements (e.g., silver halide crystals) which are usefully converted for each two-dimensional data slice. Moreover, careful control over various process parameters, including the coherence, polarization, and scattering of the laser beam, as well as the exposure time and the grey level value of the data, permit each individual hologram comprising the master hologram to consume (convert) a quantity of silver halide crystals within the emulsion in proportion to, among other things, the number of data slices comprising the data set.
In accordance with yet a further aspect of the invention, a hologram viewing device is provided for viewing the hologram produced in accordance with the invention. In particular, an exemplary viewing box in accordance with the present invention comprises a suitably enclosed, rectangular apparatus comprising a broad spectrum light source, e.g., a white light source mounted therein, a collimating (e.g., Fresnel) lens, a diffraction grating, and a Venetian blind (louver). The collimating lens is configured to direct a collimated source of white light through the diffraction grating. In the context of the present invention, a collimated light refers to light in which all components thereof have the same direction of propagation such that the beam has a substantially constant cross-sectional area over a reasonable propagation length.
The diffraction grating is configured to pass light therethrough at an angle which is a function of the wavelength of each light component. The hologram also passes light therethrough at respective angles which are a function of the corresponding wavelengths. By inverting the hologram prior to viewing, all wavelengths of light thus emerge from the hologram with respect to the grating substantially orthogonally thereto.